JDBC application Performance mystry

I programmed a Java application that read from data files and update Database through JDBC-ODBC. Normally it was quite slow, 200 insert/delete takes about 50 seconds. Might have been low resource on my system. But no matter if I close all other application, it still run roughly 50 second, +/- 5 sec. I didn't close the application and left over-night. Now I run the same 200 insert/delete testing operation set again, 2 seconds! repeatedly 2 second. What is going on?!?!